Description:
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The htmlentities function returns an empty string when used on a null value. This might seem irrelevant, however the htmlentities is used regularly to escape data before using it in a database query. A null value might be needed in such queries, and it would be preferred if it does not need to be considered a special case if a part of an data escaping routine.

For example, the Zend_Filter_Input component of the Zend_Framework uses htmlentities as the default escape filter. Passing null values in an escape routing would result in null values erroneously being converted to empty strings. 

Reproduce code:
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var_dump(htmlentities(null));

Expected result:
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NULL

Actual result:
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string(0) ""